Make particle system using settings on every faces of a mesh
I figured it out by using some settings in the particle system itself(If better result, please share it with me).

 

I assigned the same particle system to both planes here, small one got 1 face and big one got 4 faces as big as the small plane.

I cranked up the Hair number to 50k but assigned only 500 per face. So both planes got the same hair density.